Cryptographically signed filesystem

ABSTRACT

A cryptographically signed filesystem provides a central database resident on a server that contains database objects. The server creates startup software to be installed in a client system&#39;s read only memory. The startup software contains a hash value for a second stage loader. The server also creates software for a bootstrap loader object which typically contains the operating system for a client system and also the bootstrap loader&#39;s hash value and a digital signature that is unique to the server. A root filesystem object is also created containing operational code and data for the client system&#39;s functionality. A hash table file is stored in the bootstrap loader that contains the names of each file in the root filesystem along with their corresponding hash values. The startup software and objects created by the server are initially installed on a client device at the time of manufacture. The client performs a staged bootup sequence that confirms that the hash value is correct for each of the objects before they are executed. The digital signature of the bootstrap loader is verified. If a hash value or digital signature is incorrect, the client device is disabled or the file deleted or replaced. The server can update a client&#39;s bootstrap loader and root filesystem at any time through the transmission of slices.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-part of U.S. patent application Ser. No. 09/422,139, filed on 20 Oct. 1999 now U.S. Pat. No. 6,728,713; and claims benefit of U.S. Provisional Patent Application Ser. No. 60/303,179, filed on 5 Jul. 2001.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to the storing and viewing of television program material in a computer environment. More particularly, the invention relates to the secure storage, distribution, and maintenance of information in a distributed self-maintaining database management system in a computer environment.

2. Description of the Prior Art

A classic tension exists in the design of automated data processing systems between pure client-server based systems, such as computer mainframe systems or the World Wide Web, and pure distributed systems, such as Networks of Workstations (NOWS) that are used to solve complex computer problems, such as modeling atomic blasts or breaking cryptographic keys.

Client-server systems are popular because they rely on a clean division of responsibility between the server and the client. The server is often costly and specially managed, since it performs computations or stores data for a large number of clients. Each client is inexpensive, having only the local resources needed to interact with the user of the system. A network of reasonable performance is assumed to connect the server and the client. The economic model of these systems is that of centralized management and control driving down the incremental cost of deploying client systems.

However, this model has significant costs that must be considered. For instance, the incremental cost of adding a new client system may be quite high. Additional network capacity must be available, sufficient computing resources must be available to support that client, including storage, memory and computing cycles, and additional operational overhead is needed for each client because of these additional resources. As the central servers become larger and more complex they become much less reliable. Finally, a system failure of the server results in all clients losing service.

Distributed systems are popular because the resources of the system are distributed to each client, which enables more complex functionality within the client. Access to programs or data is faster since they are located with the client, reducing load on the network itself. The system is more reliable, since the failure of a node affects only it. Many computing tasks are easily broken down into portions that can be independently calculated, and these portions are cheaply distributed among the systems involved. This also reduces network bandwidth requirements and limits the impact of a failed node.

On the other hand, a distributed system is more complex to administer, and it may be more difficult to diagnose and solve hardware or software failures.

Television viewing may be modeled as a client-server system, but one where the server-to-client network path is for all intents and purposes of infinite speed, and where the client-to-server path is incoherent and unmanaged. This is a natural artifact of the broadcast nature of television. The cost of adding another viewer is zero, and the service delivered is the same as that delivered to all other viewers.

There have been, and continue to be, many efforts to deliver television programming over computer networks, such as the Internet, or even over a local cable television plant operating as a network. The point-to-point nature of computer networks makes these efforts unwieldy and expensive, since additional resources are required for each additional viewer. Fully interactive television systems, where the viewer totally controls video streaming bandwidth through a client settop device, have proven even more uneconomical because dedication of server resources to each client quickly limits the size of the system that can be profitably built and managed.

However, television viewers show a high degree of interest in choice and control over television viewing. Currently, the majority of Digital Video Recorders (DVR) are client settop devices that provide television viewers with the choice and control over television viewing that they desire. These are products that receive and record to a hard drive, a large amount of audio/video content (television broadcasts, satellite-TV broadcasts, cable-TV broadcasts, and direct downloads of program materials via the Internet).

DVR systems may receive and store A/V content which is of substantial commercial value—first-run and pay-per-view movies, special sporting events, and so forth. The right to record and view such material is usually granted under a license of some sort, and such use-licenses typically have limits. The person buying a use-license to such material is typically not granted permission to make or distribute copies of the material, and in some cases is not permitted to record and replay (“time-shift”) the material at all. In many DVR products, the right to use the DVR features themselves (e.g., program guides, programmed or manual recordings, time-shifting, etc.) is subject to a monthly or other service fee.

Because of the commercial value of this content, DVR systems may be an attractive target for people who seek to avoid the licensing and service fees, or who wish to make copies of the content recorded on the system. Many of the methods by which subversion can be accomplished, involve making changes to the software and data stored on the DVR. A “pirate” might, for example, try to modify or “patch” the DVR software so that it would allow recording of “non-recordable” programs, preserve recordings past the expiration date specified in a viewing license, or access the DVR software features without receiving a “your bill has been paid and your account is up-to-date” authorization from the DVR service provider. A pirate might also wish to install new software which would “read back” a recorded program from the DVR's disk, and transfer the content to another disk or to another computer so that the content could be posted on the Internet or used to manufacture unauthorized DVD or Video CD images.

In order to deter such attacks, it is necessary to prevent the DVR from executing any software which has been modified by an unauthorized party. In order to do this, the DVR must have a way of examining each piece of software it is asked to execute and determine with high precision whether the software is “authorized and not modified”.

It would be advantageous to provide a cryptographically signed filesystem that enables a client system to check if valid software and data are being used with high reliability and acceptable performance. It would further be advantageous to provide a cryptographically signed filesystem that enables a server to create secure software and data packages for transmission to client systems.

SUMMARY OF THE INVENTION

The invention provides a cryptographically signed filesystem. The system enables a client system to check if valid software and data are being used with high reliability and acceptable performance. In addition, the invention provides a system enables a server to create secure software and data packages for transmission to client systems.

The invention provides a central database resident on a server that contains database objects. Objects to be replicated are gathered together into distribution packages called slices. A slice is a subset of the central database which is relevant to clients within a specific domain, such as a geographic region, or under the footprint of a satellite transmitter.

Using standard, currently existing techniques, ranging from private data channels in digital television signals, through modulation of data onto the Vertical Blanking Interval (VBI) of an analog television signal, to direct connection with the server using a modem, slices are transmitted to the client devices, which choose portions of the information to save locally.

The server creates startup software to be installed in a client system's read only memory. The startup software contains a hash value for a second stage loader. The client uses the hash value to confirm the validity of the second stage loader software.

The server also creates software for a bootstrap loader object which typically contains the operating system for the client system. The bootstrap loader file contains the bootstrap loader's hash value and a digital signature that is unique to the server. A root filesystem object is also created containing operational code and data for the client system's functionality. A hash table file is stored in the bootstrap loader that contains the names of each file in the root filesystem along with their corresponding hash values.

The startup software and objects created by the server are initially installed on the client devices at the time of manufacture.

The client system performs a staged bootup sequence using the startup software stored in read only memory. The startup software confirms that the hash value is correct for the stored second stage loader. If the hash value is incorrect, the client device is disabled. Otherwise, the second stage loader is executed. The second stage loader then verifies that the hash value and digital signature of the bootstrap loader are correct. If either is invalid, then the client system is disabled. If both are valid, then control is transferred to the bootstrap loader.

The bootstrap loader then scans the root filesystem stored on the client device to verify that all of the files in the root filesystem have an entry in the corresponding hash table file. If any files do not have a matching entry in the hash table file, then the client system can either discard the non-matching files or replace the files with correct copies.

The server can update a client's bootstrap loader and root filesystem at any time through the transmission of slices.

Other aspects and advantages of the invention will become apparent from the following detailed description in combination with the accompanying drawings, illustrating, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block schematic diagram of a preferred embodiment of a distributed television viewing management system according to the invention;

FIG. 2 is a block schematic diagram of the structure of a viewing object in computer storage for programmatic access according to the invention;

FIG. 3 is a block schematic diagram showing how the schema for a viewing object is structured in computer storage for programmatic access according to the invention;

FIG. 4 is a block schematic diagram showing an example graph of relationships between viewing objects which describe information about programs according to the invention;

FIG. 5 is a block schematic diagram showing an example graph of relationships generated when processing viewer preferences to determine programs of interest according to the invention;

FIG. 6 is a block schematic diagram showing the scheduling of inputs and storage space for making recordings according to the invention;

FIG. 7 is a flowchart showing the steps taken to schedule a recording using the mechanism depicted in FIG. 6 according to the invention;

FIG. 8 is a block schematic diagram of a preferred embodiment of the invention showing the bootstrap system configuration according to the invention;

FIG. 9 a is a block schematic diagram of the decision flowchart for the bootstrap component according to the invention;

FIG. 9 b is a block schematic diagram of the decision flowchart for the bootstrap component according to the invention;

FIG. 10 is a block schematic diagram of the decision flowchart for the software installation procedure according to the invention;

FIG. 11 is a block schematic diagram of a flowchart showing the steps taken to create a cryptographically signed filesystem according to the invention;

FIG. 12 a is a block schematic diagram of the decision flowchart for the bootstrap component performing hash and digital signature verification according to the invention; and

FIG. 12 b is a block schematic diagram of the decision flowchart for the bootstrap component performing hash and digital signature verification according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is embodied in a cryptographically signed filesystem. A system according to the invention enables a client system to check if valid software and data are being used with high reliability and acceptable performance. In addition, the invention provides a system that enables a server to create secure software and data packages for transmission to client systems.

The invention is embodied in a television viewing information transmission and collection system that improves the ability of the individual viewer to select and automatically timeshift television programs while providing opportunities for a service provider to enhance and direct the viewing experience. The invention describes a system which is fully distributed, in that calculations pertaining to an individual viewer are performed personally for that viewer within a local client device, while providing for the reliable aggregation and dissemination of information concerning viewing habits, preferences or purchases.

The Database of Television Viewing Information

FIG. 1 gives a schematic overview of the invention. Central to the invention is a method and apparatus for maintaining a distributed database of television viewing information among computer systems at a central site 100 and an extremely large number of client computing systems 101. The process of extracting suitable subsets of the central copy of the database is called “slicing” 102, delivering the resulting “slices” to clients is called “transmission” 103, delivering information collected about or on behalf of the viewer to the central site is called “collection” 104, and processing the collected information to generate new television viewing objects or reports is called “analysis” 107; in all cases, the act of recreating an object from one database within another is called “replication” 105. Data items to be transmitted or collected are termed “objects” 106, and the central database and each replicated subset of the central database contained within a client device is an “object-based” database. The objects within this database are often termed “television viewing objects”, “viewing objects”, or simply “objects”, emphasizing their intended use. However, one skilled in the art will readily appreciate that objects can be any type of data.

The viewing object database provides a consistent abstract software access model for the objects it contains, independent of and in parallel with the replication activities described herein. By using this interface, applications may create, destroy, read, write and otherwise manipulate objects in the database without concern for underlying activities and with assurance that a consistent and reliable view of the objects in the database and the relationships between them is always maintained.

Basic Television Viewing Object Principles

Referring to FIG. 2, television viewing objects are structured as a collection of “attributes” 200. Each attribute has a type 201, e.g., integer, string or boolean, and a value 202. All attribute types are drawn from a fixed pool of basic types supported by the database.

The attributes of an object fall into two groups: “basic” attributes, which are supplied by the creator or maintainer of the viewing object; and “derived” attributes, which are automatically created and maintained by mechanisms within the database. Basic attributes describe properties of the object itself; derived attributes describe the relationships between objects. Basic attributes are replicated between databases, whereas derived attributes are not.

With respect to FIG. 3, there is a small set of fundamental object types defined by the invention; each object type is represented as a specific set of related attributes 300, herein called a “schema”. The schema defines a template for each attribute type 301, which includes the type 302 and name of the attribute 303. Actual television viewing objects are created by allocating resources for the object and assigning values to the attributes defined by the schema. For example, a “program” schema might include attributes such as the producer, director or actors in the program, an on-screen icon, a multi-line description of the program contents, an editorial rating of the program, etc. A physical program object is created by allocating storage for it, and filling in the attributes with relevant data.

There is one special object type predefined for all databases called the schema type. Each schema supported by the database is represented by a schema object. This allows an application to perform “introspection” on the database, i.e., to dynamically discover what object types are supported and their schema. This greatly simplifies application software and avoids the need to change application software when schemas are changed, added or deleted. Schema objects are handled the same as all other viewing objects under the methods of this invention.

Referring again to FIG. 2, each object in a database is assigned an “object ID” 203 which must be unique within the database. This object ID may take many forms, as long as each object ID is unique. The preferred embodiment uses a 32-bit integer for the object ID, as it provides a useful tradeoff between processing speed and number of unique objects allowed. Each object also includes a “reference count” 204, which is an integer giving the number of other objects in the database which refer to the current object. An object with a reference count of zero will not persist in the database (see below).

One specific type of viewing object is the “directory” object. A directory object maintains a list of object IDs and an associated simple name for the object. Directory objects may include other directory objects as part of the list, and there is a single distinguished object called the “root” directory. The sequence of directory objects traversed starting at the root directory and continuing until the object of interest is found is called a “path” to the object; the path thus indicates a particular location within the hierarchical namespace created among all directory objects present in the database. An object may be referred to by multiple paths, meaning that one object may have many names. The reference count on a viewing object is incremented by one for each directory which refers to it.

Methods for the Maintenance of Database Consistency and Accuracy

One of the features of a preferred embodiment of the invention is to insure that each database replica remains internally consistent at all times, and that this consistency is automatically maintained without reference to other databases or the need for connection to the central site. There is no assurance that transmission or collection operations happen in a timely manner or with any assured periodicity. For instance, a client system may be shut off for many months; when a transmission to the system is finally possible, the replication of objects must always result in a consistent subset of the server database, even if it is not possible to transmit all objects needed to bring the central and client databases into complete synchronization.

Even more serious, there can be no guarantee of a stable operational environment while the database is in use or being updated. For example, electrical power to the device may cease. This invention treats all database updates as “transactions”, meaning that the entire transaction will be completed, or none of it will be completed. The specific technique chosen is called “two-phase commit”, wherein all elements of the transaction are examined and logged, followed by performing the actual update. One familiar in the art will appreciate that a standard journaling technique, where the transaction is staged to a separate log, combined with a roll-forward technique which uses the log to repeat partial updates that were in progress when the failure occurred, is sufficient for this purpose.

One required derived attribute of every object is the “version”, which changes with each change to the object; the version attribute may be represented as a monotonically increasing integer or other representation that creates a monotonic ordering of versions. The schema for each object that may be replicated includes an attribute called “source version” which indicates the version of the object from which this one was replicated.

Transmission of a viewing object does not guarantee that every client receives that object. For instance, while the object is being broadcast, external factors such as sunspots, may destroy portions of the transmission sequence. Viewing objects may be continually retransmitted to overcome these problems, meaning that the same object may be presented for replication multiple times. It is inappropriate to simply update the database object each time an object to be replicated is received, as the version number will be incremented although no change has actually occurred. Additionally, it is desirable to avoid initiating a transaction to update an object if it is unnecessary; considerable system resources are consumed during a transaction.

Two approaches are combined to resolve this problem. First, most objects will have a basic attribute called “expiration”. This is a date and time past which the object is no longer valid, and should be discarded. When a new object is received, the expiration time is checked, and the object discarded if it has expired. Expiration handles objects whose transmission is delayed in some fashion, but it does not handle multiple receptions of the same unexpired object.

The source version attribute handles this problem. When a viewing object is transmitted, this attribute is copied from the current version attribute of the source object. When the viewing object is received, the source version of the received object is compared with the source version of the current object. If the new object has a higher source version attribute, it is copied over the existing object, otherwise it is discarded.

It is assumed that a much greater number of viewing objects are transmitted than are of interest to any particular client system. For example, a “channel” viewing object which describes the channels on a particular cable system is of no interest to clients attached to other cable systems. Because of the overhead of capturing and adding new objects to the database, it would be advantageous for received objects to be filtered on other attributes in addition to those described above. The invention accomplishes this by using a filtering process based on object type and attribute values. In one implementation, this filtering process is based on running executable code of some kind, perhaps as a sequence of commands, which has been written with specific knowledge of various object types and how they should be filtered.

In a preferred embodiment of the invention, a “filter” object is defined for each object type which indicates what attributes are required, should not be present, or ranges of values for attributes that make it acceptable for addition to the database. One skilled in the art will readily appreciate that this filter object may contain executable code in some form, perhaps as a sequence of executable commands. These commands would examine and compare attributes and attribute values of object being filtered, resulting in an indication of whether the object should be the subject of further processing.

Viewing objects are rarely independent of other objects. For example, a “showing” object (describing a specific time on a specific channel) is dependent on a “program” object (describing a specific TV program). One important aspect of maintaining consistency is to insure that all dependent objects either already exist in the database or are to be added as part of a single transaction before attempting to add a new viewing object. This is accomplished using a basic attribute of the new viewing object called the “dependency” attribute, which simply lists the object IDs and source versions of objects that the new object is dependent on. Clearly, new versions of an object must be compatible, in the sense that the schema defining new versions be the same or have a strict superset of the attributes of the original schema.

When a new viewing object is received, the database is first checked to see if all dependencies of that object are present; if so, the object is added to the database. Otherwise, the new object is “staged”, saving it in a holding area until all dependent objects are also staged. Clearly, in order for a new set of viewing objects to be added to the database, the dependency graph must be closed between objects in the staging area and objects already existing in the database, based on both object ID and source version. Once closure is achieved, meaning all dependent objects are present, the new object(s) are added to the database in a single atomic transaction.

Naming and Finding Television Viewing Objects

Directory objects have been described previously. Referring to FIG. 4, the collection of directory objects, and the directed graph formed by starting at the root path 400 and enumerating all possible paths to viewing objects is called a “namespace”. In order for an object to be found without knowing a specific object ID, one or more paths within this namespace must refer to it. For instance, application software has little interest in object IDs, instead the software would like to refer to objects by paths, for instance “/tvschedule/today”. In this example, the actual object referred to may change every day, without requiring changes in any other part of the system.

One way in which a path to an object may be established is by specifying a “pathname” basic attribute on the object. The object is added to the database, and directory objects describing the components of the path are created or updated to add the object. Such naming is typically used only for debugging the replication mechanisms. Setting explicit paths is discouraged, since the portions of the central database replicated on each client system will be different, leading to great difficulty in managing pathnames among all replicas of the database.

A preferred method for adding an object to the database namespace is called “indexing”. In a preferred embodiment of the invention, an “indexer” object is defined for each object type which indicates what attributes are to be used when indexing it into the database namespace. One skilled in the art will readily appreciate that this indexer object may contain executable code in some form, perhaps as a sequence of executable commands. These commands would examine and compare attributes and attribute values of object being indexed, resulting in an indication of where the object should be located in the namespace.

Based on the object type, the indexer examines a specific set of attributes attached to the object. When such attributes are discovered the indexer automatically adds a name for the object, based on the value of the attribute, within the hierarchical namespace represented by the graph of directories in the database. Referring again to FIG. 4, a program object may have both an “actor” attribute with value “John Wayne” and a “director” attribute with value “John Ford” 401. The root directory might indicate two sub-directories, “byactor” 402 and “bydirector” 403. The indexer would then add the paths “/byactor/John Wayne” and “/bydirector/John Ford” to the database, both of which refer to the same object 401.

A derived attribute is maintained for each object listing the directory objects which refer to this object 404. As the indexer adds paths to the namespace for this object, it adds the final directory ID in the path to this list. This insures closure of the object graph—once the object has been found, all references to that object within the database are also found, whether they are paths or dependencies.

This unique and novel method of adding objects to the database has significant advantages over standard approaches. The indexer sorts the object into the database when it is added. Thus, the search for the object associated with a particular path is a sequence of selections from ordered lists, which can be efficiently implemented by one familiar with the art.

Deleting Objects from the Database

While the rules for adding objects to the database are important, the rules for removing objects from the database are also important in maintaining consistency and accuracy. For example, if there were no robust rules for removing objects, the database might grow unboundedly over time as obsolete objects accumulate.

The cardinal rule for deleting objects from the database is based on reference counting; an object whose reference count drops to zero is summarily deleted. For instance, this means that an object must either be referred to by a directory or some other object to persist in the database. This rule is applied to all objects in the closed dependency graph based on the object being deleted. Thus, if an object which refers to other objects (such as a directory) is deleted, then the reference count on all objects referred to is decremented, and those objects similarly deleted on a zero count, and so forth.

There is also an automatic process which deletes objects from the database called the “reaper”. Periodically, the reaper examines all objects in the database, and depending on the object type, further examines various attributes and attribute values to decide if the object should be retained in the database. For example, the expiration attribute may indicate that the object is no longer valid, and the reaper will delete the object.

In the preferred embodiment, using a method similar to (or perhaps identical to) the filtering and indexing methods described above, the reaper may instead access a reaper object associated with the object type of the current object, which may contain executable code of various kinds, perhaps a sequence of executable commands. This code examines the attributes and attribute values of the current object, and determines if the object should be deleted.

The overhead of individually deleting every object for which the reference count has been decremented to zero may be quite high, since every such deletion results in a transaction with the database. It would be advantageous to limit the performance impact of reaping objects, such that foreground operations proceed with maximum speed. In a preferred embodiment, this is accomplished using a technique based on common garbage collection methods.

For instance, instead of deleting an object whose reference count has been decremented to zero, the reaper performs no other action. Periodically, a background task called the garbage collector examines each object in the database. If the object has a reference count of zero, it is added to a list of objects to be deleted. In one embodiment, once the garbage collector has examined the entire database, it would delete all such objects in a single transaction. One familiar in the art will appreciate that this method may also result in a significant performance penalty, as other accesses to the database may be delayed while the objects are being deleted. In addition, if all objects are to be properly deleted, changes to the database may have to be delayed while the garbage collector is active, resulting in even worse performance.

In a preferred embodiment, the garbage collector examines the database in a series of passes. Once a specific number of objects has been collected, they are deleted in a single transaction. Said process continues until all objects have been examined. This technique does not guarantee that all garbage objects are collected during the examination process, since parallel activities may release objects previously examined. These objects will be found, however, the next time the garbage collector runs. The number of objects deleted in each pass is adjustable to achieve acceptable performance for other database activities.

Operations on the Distributed Television Viewing Object Database

Considerations in Maintaining the Distributed Viewing Object Database

The replication of television viewing objects among the instances of the distributed database necessarily requires the transmission of objects over unreliable and unsecure distribution channels.

For example, if the objects are transmitted over a broadcast mechanism, such as within a radio or television transmission, there can be no assurance that the data is transmitted accurately or completely. Weather, such as rainstorms, may cause dropouts in the transmission. Other sources of interference may be other broadcast signals, heavy equipment, household appliances, etc.

One skilled in the art will readily appreciate that there are standard techniques for managing the transmission of data over unreliable channels, including repeated transmissions, error correcting codes, and others, which may be used for transmission, any or all of which may be used in any particular instance.

For efficiency, objects to be replicated are gathered together into distribution packages, herein called “slices”. A slice is a subset of the television viewing object database which is relevant to clients within a specific domain, such as a geographic region, or under the footprint of a satellite transmitter.

Security of these slices is quite important. Slices are used to add objects to the database which are used to provide valuable services to users of the database, as well as to store information that may be considered private or secret. Because of the broadcast-oriented nature of slice transmission, slices may be easily copied by third parties as they are transmitted. A practical solution to these problems is to encrypt the slice during transmission. An ideal reference text on the techniques employed in the invention is “Applied Cryptography: Protocols, Algorithms, and Source Code in C” by Bruce Schneier, John Wiley and Sons, 1995.

In a preferred embodiment of the invention, a secure, encrypted channel is established using techniques similar to those described in U.S. Pat. Ser. No. 4,405,829, often described as asymmetric key encryption, or sometimes public/private key pair encryption. A practitioner skilled in the art will recognize that protocols based on asymmetric key encryption serves as a reliable and efficient foundation for authentication of client devices and secure distribution of information. In general, authentication is provided using an exchange of signed messages between the client and central systems. Secure distribution is provided by encrypting all communications using a short-lived symmetric key sent during an authentication phase.

Successful security requires that sender and receiver agree beforehand on the asymmetric key pair to be used for encryption. Such key distribution is the weakest link in any cryptographic system for protecting electronic data. U.S. Pat. No. 6,385,739, entitled “Self-Test Electronic Assembly and Test System,” also owned by the Applicant, describes a mechanism whereby the client device generates the asymmetric key pair automatically as the final step in the manufacturing process. The private key thus generated is stored within a secure microprocessor embedded within the client device, such that the key is never presented to external devices. The public key thus generated is transmitted to a local manufacturing system, which records the key along with the client serial number in a secure database. This database is later securely transmitted to the central distribution system, where it is used to perform secure communications with the client.

This unique and novel application of key generation solves the problem of key distribution, as the private key is never presented to external components in the client, where it might be discerned using special tools, such as a logic analyzer. Instead, it may only be used within the security microprocessor itself to decrypt messages originally encrypted with the public key, the results of which are then provided to external components.

The remainder of this discussion assumes that all communications between client and central systems are authenticated and encrypted as described above.

Transmitting Viewing Objects to the Client Systems

Referring again to FIG. 1, in a preferred embodiment of the invention the following steps constitute “transmission” of television viewing objects from the central database using slices:

-   1. There may be many mechanisms for transmitting slices to the     universe of client viewing devices. For instance, the slices may be     directly downloaded over a telephone modem or cable modem 109, they     may be modulated into lines of the Vertical Blanking Interval (VBI)     of a standard television broadcast 108 , or added to a digital     television multiplex signal as a private data channel. One skilled     in the art will readily appreciate that any mechanism which can     transmit digital information may be used to transmit slices of the     television viewing object database.     -   The first step in preparing television viewing objects for         transmission is recognizing the transmission mechanism to be         used for this particular instance, and creating a slice of a         subset of the database that is customized for that mechanism.         For example, the database may contain television viewing objects         relating to all programs in the country. However, if television         viewing objects are to be sent using VBI modulation on a local         television signal, only those television viewing objects         relating to programs viewable within the footprint of the         television broadcast being used to carry them should be         contained within the relevant slice. Alternatively, if some of         the television viewing objects contain promotional material         related to a particular geographic region, those objects should         not be transmitted to other geographic regions.     -   In a preferred embodiment of the invention, the speed and         periodicity of traversing the database and generating slices for         transmission is adjustable in an arbitrary fashion to allow         useful cost/performance tradeoffs to be made. For instance, it         may only be necessary to create slices for certain transmission         methods every other day, or every hour.     -   The final step in preparing each slice is to encrypt the slice         using a short-lived symmetric key. Only client devices which         have been authenticated using secure protocols will have a copy         of this symmetric key, making them able to decrypt the slice and         access the television viewing objects within it. -   2. Once a slice is complete, it is copied to the point at which the     transmission mechanism can take and send the data 110. For telephone     connections, the slice is placed on a telephony server 111 which     provides the data to each client as it calls in. If television     broadcast is used, the slice is copied onto equipment co-resident     with the station television transmitter, from whence it is modulated     onto the signal. In these and similar broadcast-oriented cases, the     slice is “carouseled”, i.e., the data describing the slice is     repeated continually until a new slice is provided for transmission.     -   This repetitive broadcast of slices is required because there         can be no assurance that the signal carrying the data arrives         reliably at each client. The client device may be powered off,         or there may be interference with reception of the signal. In         order to achieve a high degree of probability that the         transmitted slices are properly received at all client devices,         they are continually re-broadcast until updated slices are         available for transmission.     -   A preferred embodiment of the invention uses broadcast         mechanisms such as a television signal to transmit the slice.         However, it is desirable to provide for download over a         connection-based mechanism, such as a modem or Internet         connection. Using a connection-based mechanism usually results         in time-based usage fees, making it desirable to minimize the         time spent transmitting the slice.     -   This is accomplished using a two-step process. When the         connection is established, the client system sends an inventory         of previously received slices to telephony servers 111. The         server compares this inventory with the list of slices that         should have been processed by that client. Slices which were not         processed are transmitted to the client system. -   3. The slice is transmitted by breaking the encrypted slice into a     succession of short numbered data packets. These packets are     captured by client systems and held in a staging area until all     packets in the sequence are present. The packets are reassembled     into the slice, which is then decrypted. The television viewing     objects within the slice are then filtered for applicability,     possibly being added to the local television viewing object     database. This process replicates a portion of the central database     of television viewing objects reliably into the client.     -   The invention keeps track of the time at which data packets are         received. Data packets which are older than a selected time         period are purged from the staging area on a periodic basis;         this avoids consuming space for an indefinite period while         waiting for all parts of a slice to be transmitted.     -   Especially when transmitting the objects over a broadcast         medium, errors of various kinds may occur in the transmitted         data. Each data packet is stamped with an error detecting code         (a parity field or CRC code, for example). When an error is         detected the data packet is simply discarded. The broadcast         carousel will eventually retransmit the data packet, which is         likely to be received properly. Slices of any size may thus be         sent reliably; this is achieved at the cost of staging received         portions of the object on the client until all portions are         properly received. -   4. There may be one or more “special” slices transmitted which     communicate service related data to the client system, particularly     service authorization information. It is important that the service     provider be able to control the client system's access to premium     services if the viewer has failed to pay his bill or for other     operational reasons.     -   One particular type of special slice contains an “authorization”         object. Authorization objects are generally encrypted using         asymmetric key encryption based on the public/private key pair         associated with a specific client. If the slice can be         successfully decrypted by the security microprocessor using the         embedded private key, the slice will contain an object         indicating the allowable time delay before another authorization         object is received, as well as one or more symmetric keys valid         for a short time period. The delay value is used to reset a         timestamp in the database indicating when the client system will         stop providing services. The symmetric keys are stored in the         local television viewing object database, to be used in         decrypting new slices which may be received.     -   If the client has not received a proper authentication object by         the time set in the database, it will commence denial of most         services to the viewer (as specified by the service provider).         Also contained within an authentication object are one or more         limited-lifetime download keys which are needed to decrypt the         slices that are transmitted. Clearly, if a client system is         unable to authenticate itself, it will not be able to decrypt         any objects.     -   Each authorization slice is individually generated and         transmitted. If broadcast transmission is used for the slices,         all relevant authorizations are treated identically to all other         slices and carouseled along with all other data. If direct         transmission is used, such as via a phone connection, only the         authentication slice for that client is transmitted. -   5. Once the client device has received a complete database slice, it     uses the methods described earlier to add the new object contained     within it to the database.     Collecting Information from the Client Systems

Referring again to FIG. 1, in a preferred embodiment of the invention the following steps constitute “collection” of television viewing objects from each client database:

-   1. As the viewer navigates the television channels available to him,     the client system records interesting information, such as channel     tuned to, time of tuning, duration of stay, VCR-like actions (e.g.,     pause, rewind), and other interesting information. This data is     stored in a local television viewing object.     -   Additionally, the viewer may indicate interest in offers or         promotions that are made available, or he may indicate a desire         to purchase an item. This information is also recorded into a         local television viewing object.     -   Additionally, operation of the client device may result in         important data that should be recorded into a television viewing         object. For example, errors may occur when reading from the hard         disk drive in the client, or the internal temperature of the         device may exceed operational parameters. Other similar types of         information might be failure to properly download an object,         running out of space for various disk-based operations, or rapid         power cycling. -   2. At a certain time, which may be immediate or on a periodic basis,     the client system contacts the central site via a direct connection     104 (normally via phone and/or an Internet connection). The client     device sends a byte sequence identifying itself which is encrypted     with its secret key. The server fetches the matching television     viewing object for the client device from the database, and uses the     key stored there to decrypt the byte sequence. At the same time, the     server sends a byte sequence to the client, encrypted in its secret     key, giving the client a new one-time encryption key for the     session.     -   Both sides must successfully decrypt their authentication         message in order to communicate. This two-way handshake is         important, since it assures both client and server that the         other is valid. Such authentication is necessary to avoid         various attacks that may occur on the client system. For         example, if communications were not authenticated in such a         fashion, a malicious party might create an “alias” central site         with a corrupt television viewing object database and provide         bad information to a client system, causing improper operation.         All further communication is encrypted using the one-time         session key. Encrypted communication is necessary because the         information may pass across a network, such as the Internet,         where data traffic is open to inspection by all equipment it         passes through. Viewing objects being collected may contain         information that is considered private, so this information must         be fully protected at all times.     -   Assuming that the authentication phase is successful, the two         parties treat the full-duplex phone line as two one-way         broadcast channels. New slices are delivered to the client, and         viewing data to be collected is sent back. The connection is         ended when all data is delivered.     -   One skilled in the art will readily appreciate that this         connection may take place over a network, such as the Internet         running standard TCP/IP protocols, transparently to all other         software in the system. -   3. Uploaded information is handled similarly by the server; it is     assumed to represent television viewing objects to be replicated     into the central database. However, there may be many uploaded     viewing objects, as there may be many clients of the service.     Uploaded objects are therefore assigned a navigable attribute     containing information about their source; the object is then     indexed uniquely into the database namespace when it is added.     -   Uploaded viewing objects are not immediately added to the         central database; instead they are queued for later insertion         into the database. This step allows the processing of the queue         to be independent of the connection pattern of client devices.         For instance, many devices may connect at once, generating a         large number of objects. If these objects were immediately added         to the central database, the performance of all connections         would suffer, and the connection time would increase. Phone         calls are charged by duration, thus any system in which         connection time increases as a function of load is not         acceptable.     -   Another advantage of this separation is that machine or network         failures are easily tolerated. In addition, the speed at which         viewing objects are processed and added to the central database         may be controlled by the service provider by varying the         computer systems and their configurations to meet cost or         performance goals.     -   Yet another advantage of this separation is that it provides a         mechanism for separating data collected to improve service         operations and data which might identify an individual viewer.         It is important that such identifying data be kept private, both         for legal reasons and to increase the trust individuals have in         the service. For instance, the navigable attribute assigned to a         viewing object containing the record of a viewer's viewing         choices may contain only the viewer's zip code, meaning that         further processing of those objects can construct no path back         to the individual identity.     -   Periodic tasks are invoked on the server to cull these objects         from the database and dispose of them as appropriate. For         example, objects indicating viewer behavior are aggregated into         an overall viewer behavior model, and information that might         identify an individual viewer is discarded. Objects containing         operational information are forwarded to an analysis task, which         may cause customer service personnel to be alerted to potential         problems. Objects containing transactional information are         forwarded to transaction or commerce systems for fulfillment.     -   Any of these activities may result in new television viewing         objects being added to the central database, or in existing         objects being updated. These objects will eventually be         transmitted to client devices. Thus, the television viewing         management system is closed loop, creating a self-maintaining         replicated database system 105 which can support any number of         client systems.         Processing of Television Viewing Objects by Client Systems

Television viewing objects may contain the following types of information: television program descriptions and showing times; cable, satellite or broadcast signal originator information, such as channel numbering and identification; viewer preference information, such as actors, genre, showing times, etc.; software, such as enhanced database software, application software, operating system software, etc.; statistical modeling information such as preference vectors, demographic analysis, etc.; and any other arbitrary information that may be represented as digital data.

Methods Applied to Program Guide Objects

Program guide objects contain all information necessary for software running in the client system to tune, receive, record and view programs of interest to the user of the client system, selecting from among all available programs and channels as described by objects within the database.

This program guide information is updated on a regular basis by a service provider. This is handled by the provider acquiring program guide information in some manner, for instance, from a commercial supplier of such information or other sources of broadcast schedule information. This data is then processed using well-understood software techniques to reduce the information to a collection of inter-related viewing objects.

Referring again to FIG. 4, a typical relationship between program guide objects is shown. A television “network” object 407 is any entity which schedules and broadcasts television programming, whether that broadcast occurs over the air, cable, satellite, or other suitable medium. A television “program” object 401 is a description of any distinct segment of a television broadcast signal, such as a particular program, commercial advertisement, station promotion, opener, trailer, or any other bounded portion of a television signal. A “showing” object 406 is a portion of the broadcast schedule for a network on which a program is broadcast. A “channel map” object maps a network broadcast onto a particular broadcast channel for the medium being used; for instance, a channel map object for a satellite broadcast service would include information about the transponder and data stream containing the broadcast. Using the previously described methods, this program guide data is replicated from the central site to the client systems, where application software in the client systems use the data to manage television viewing.

The service provider may also provide aggregation viewing objects, which describe a set of program guide objects that are interrelated in some fashion. For instance, a “Star-Trek” collection might contain references to all program guide objects associated with this brand name. Clearly, any arbitrary set of programs may be aggregated in this fashion. Aggregation objects are similar to directories. For instance, the Star Trek collection might be found at “/showcases/Star Trek” in the hierarchical namespace. Aggregation objects are also program guide objects, and may be manipulated in a similar fashion, including aggregating aggregation objects, and so forth.

The client system may further refine the collection of program objects. In a system where programming may be captured to internal storage, each captured program is represented by a new program guide object, becoming available for viewing, aggregation, etc. Explicit viewer actions may also result in creation of program guide objects. For instance, the viewer may select several programs and cause creation of a new aggregation object.

This description of types of program guide objects is not meant to be inclusive; there may be many different uses and ways of generating program guide objects not herein described which still benefit from the fundamental methods of the invention.

Program guide objects are used by the application software in five ways:

-   1. In the simplest case, the viewer may wish to browse these objects     to discern current or soon-to-be-available programming. The     application software will map the object relationships described by     the database to some form of visual and audible interface that is     convenient and useful for the viewer. The viewer may indicate that a     particular program is of interest, resulting in some     application-specific action, such as recording the program to local     storage when it is broadcast. -   2. Application software may also directly process program guide     objects to choose programs that may be of interest to the viewer.     This process is typically based on an analysis of previously watched     programming combined with statistical models, resulting in a     priority ordering of all programs available. The highest priority     programs may be processed in an application specific manner, such as     recording the program to local storage when it is broadcast.     Portions of the priority ordering so developed may be presented to     the viewer for additional selection as in case 1.     -   One skilled in the art will readily appreciate that there is a         great deal of prior art centered on methods for selecting         programming for a viewer based on previous viewing history and         explicit preferences, e.g., U.S. Pat. No. 5,758,257. The methods         described in this application are unique and novel over these         techniques as they suggest priorities for the capture of         programming, not the broadcast or transmission of programming,         and there is no time constraint on when the programming may be         broadcast. Further details on these methods are given later in         this description.     -   In general, explicit viewer choices of programming have the         highest priority for capture, followed by programming chosen         using the preference techniques described herein. -   3. A client system will have a small number of inputs capable of     receiving television broadcasts or accessing Web pages across a     network such as an intranet or the Internet. A scheduling method is     used to choose how each input is tuned, and what is done with the     resulting captured television signal or Web page.     -   Referring to FIG. 6, generally, the programs of interest to the         viewer may be broadcast at any time, on any channel, as         described by the program guide objects. Additionally, the         programs of interest may be Web page Universal Resource Locators         (URL) across a network, such as an intranet or the Internet. The         channel metaphor is used to also describe the location, or URL,         of a particular Web site or page.     -   A viewer, for example, can “tune” into a Web site by designating         the Web site URL as a channel. Whenever that channel is         selected, the Web site is displayed. A Web page may also be         designated as a program of interest and a snapshot of the Web         page will be taken and recorded at a predetermined time.     -   The scheduler accepts as input a prioritized list of program         viewing preferences 603, possibly generated as per the cases         above. The scheduling method then compares this list with the         database of program guide objects, which indicate when programs         of interest are actually broadcast. It then generates a schedule         601 of time 607 versus available storage space 606 that is         optimal for the viewer's explicit or derived preferred programs.         Further details on these methods are given later in this         description. -   4. When a captured program is viewed, the matching program guide     object is used to provide additional information about the program,     overlaid on the display using any suitable technique, preferably an     On Screen Display (OSD) of some form. Such information may include,     but is not limited to: program name; time, channel or network of     original broadcast; expiration time; running time or other     information. -   5. When live programming is viewed, the application uses the current     time, channel, and channel map to find the matching program guide     object. Information from this object is displayed using any suitable     technique as described above. The information may be displayed     automatically when the viewer changes channels, when a new program     begins, on resumption of the program after a commercial break, on     demand by the viewer, or based on other conditions. -   6. Using techniques similar to those described in case 2,     application software may also capture promotional material that may     be of interest to the viewer. This information may be presented on     viewer demand, or it may be automatically inserted into the output     television signal at some convenient point. For example, an     advertisement in the broadcast program might be replaced by a     different advertisement which has a higher preference priority.     Using the time-warping apparatus, such as that described in     application Ser. No. 09/126,071, entitled “Multimedia Time Warping     System,” filed Jul. 30, 1998, it is possible to insert any stored     program into the output television signal at any point. The     time-warping apparatus allows the overlaid program to be delayed     while the stored program is inserted to make this work.     Methods for Generating a List of Preferred Programs

Viewer preferences may be obtained in a number of ways. The viewer may request that certain programs be captured, which results in the highest possible priority for those programs. Alternatively, the viewer may explicitly express preferences using appurtenances provided through the viewer interface, perhaps in response to a promotional spot for a particular program, or even during the viewing of a program. Finally, preferences may be inferred from viewing patterns: programs watched, commercial advertisements viewed or skipped, etc.

In each case, such preferences must correspond to television viewing objects stored in the replicated database. Program objects included a wealth of information about each particular program, for example: title, description, director, producer, actors, rating, etc. These elements are stored as attributes attached to a program object.

Each individual attribute may result in the generation of a preference object. Such objects store the following information:

-   1. The type of the preference item, such as actor or director     preference; -   2. The weight of the preference given by the viewer, which might be     indicated by multiple button presses or other means; -   3. The statically assigned significance of the preference in     relation to other preferences, for example, actor preference are     more significant than director preferences; -   4. The actual value of the preference item, for instance the name of     the director.

With respect to FIG. 5, preference objects are stored in the database as a hierarchy similar to that described for program guide objects, however this hierarchy is built incrementally as preferences are expressed 500. The hierarchy thus constructed is based on “direct” preferences, e.g., those derived from viewer actions or inferred preferences.

A similar hierarchy is developed based on “indirect” preferences pointing to the same preference objects 501. In general, indirect preferences are generated when preferences for aggregate objects are generated, and are used to further weight the direct preferences implied by the collection of aggregated objects. The preference objects referenced through the indirect preference hierarchy are generated or updated by enumerating the available program objects which are part of the aggregate object 502, and generating or updating preference objects for each attribute thus found.

The weight of a particular preference 503 begins at zero, and then a standard value is added based on the degree of preference expressed (perhaps by multiple button presses) or a standard value is subtracted if disinterest has been expressed. If a preference is expressed based on an aggregate viewing object, all preferences generated by all viewing objects subordinate to the aggregated object are similarly weighted. Therefore, a new weighting of relevant preference elements is generated from the previous weighting. This process is bounded by the degree of preference which is allowed to be expressed, thus all weightings fall into a bounded range.

In a preferred embodiment of the invention, non-linear combinations may be used for weighting a preference item. For instance, using statistical models provided by the central site, the client may infer that a heavily weighted preference for three attributes in conjunction indicates that a fourth attribute should be heavily weighted as well.

The list of preferred programs is generated as follows:

-   1. A table 504 is constructed which lists each possible program     object attribute, and any preference objects for that attribute that     are present are listed in that entry. -   2. It the preference item is a string, such as an actor name, a     32-bit digital signature for that string is calculated using a     32-bit CRC algorithm and stored with the table item, rather than the     string itself. This allows for much faster scanning of the table as     string comparisons are avoided, at the slight risk of two different     strings generating the same digital signature. -   3. For each program object in the database, and for each attribute     of that program, the attribute is looked up in the table. If     present, the list of preference objects for that attribute is     examined for a match with the attribute of the current program     object. If a match occurs, the weight associated with that     preference object is added to weighting associated with the program     object to generate a single weight for the program. -   4. Finally, the program objects are rank-ordered based on the     overall weighting for each program, resulting in a list of     most-preferred to leastpreferred programs.

Given this final prioritized list, a recording schedule is generated using the methods described below, resulting in a collection of recorded programs of most interest to the viewer.

Methods Applied to Scheduling Recording Versus Available Storage Space

As has been described previously, recorded programs will in general have an expiration date, after which the recorded program is removed from client storage. The viewer may at any time indicate that a program should be saved longer, which delays expiration by a viewer-selected interval. The invention views the available storage for recording programs as a “cache”; unviewed programs are removed after a time, based on the assumption they will not be watched if not watched soon after recording. Viewed programs become immediate candidates for deletion, on the assumption they are no longer interesting.

With proper scheduling of recording and deletion of old programs, it is possible to make a smaller storage area appear to be much larger, as there is an ongoing flushing of old programs and addition of new programs. Additionally, if resources are available, recordings may be scheduled of programs based on inferred preferences of the viewer; these are called “fuzzy” recordings. This results in a system where the program storage area is always “full” of programming of interest to the viewer; no program is removed until another program is recorded in its place or the viewer explicitly deletes it.

Additionally, the viewer may select a program for recording at any time, and the recording window may conflict with other scheduled recordings, or there may not be sufficient space obtainable when the program must be recorded. The invention includes unique and novel methods of resolving such conflicts.

Conflicts can arise for two reasons: lack of storage space, or lack of input sources. The television viewing system described herein includes a fixed number of input sources for recording video and a storage medium, such as a magnetic disk, of finite capacity for storing the recorded video. Recording all television programs broadcast over any significant period of time is not possible. Therefore, resolving the conflicts that arise because of resource limitations is the key to having the correct programs available for viewing.

Referring again to FIG. 6, the invention maintains two schedules, the Space Schedule 601 and the Input Schedule 602. The Space Schedule tracks all currently recorded programs and those which have been scheduled to be recorded in the future. The amount of space available at any given moment in time may be found by generating the sum of all occupied space (or space that will be occupied at that time) and subtracting that from the total capacity available to store programs. Programs scheduled for recording based on inferred preferences (“fuzzy” recordings) are not counted in this calculation; such programs automatically lose all conflict decisions.

A program may be recorded 603 if at all times between when the recording would be initiated and when it expires, sufficient space is available to hold it. In addition, for the duration of the program, there must be an input available from which to record it. The Input Schedule 602 tracks the free and occupied time slots for each input source. In a preferred embodiment of the invention, the input sources may not be used for identical services, e.g., one input may be from a digital television signal and another from an analog television signal with different programming. In this case, only those inputs from which the desired program can be recorded are considered during scheduling.

With respect to FIG. 7, a flowchart is shown describing the steps taken to schedule a recording in the preferred embodiment. First, an ordered list of showings of the program of interest are generated 701. Although a preferred embodiment of the invention orders these showings by time, such that the recording is made as soon as possible, any particular ordering might be chosen. Each showing in this list 702 is then checked to see if input 703 or space 704 conflicts occur as described above. If a showing is found with no conflicts, then the program is scheduled for recording 705.

Otherwise, a preferred embodiment of the invention selects only those showings of the program which have no input conflicts 706. Referring again to FIG. 6, one can see that over the lifetime of a recording the amount of available space will vary as other programs are recorded or expire. The list of showings is then sorted, preferably by the minimum amount of available space during the lifetime of the candidate recording. Other orderings may be chosen.

Referring again to FIG. 7, for each candidate showing, the viewer is presented with the option of shortening the expiration dates on conflicting programs 708, 709. This ordering results in the viewer being presented these choices in order from least impact on scheduled programs to greatest 707; there is no requirement of the invention that this ordering be used versus any other.

Should the viewer reject all opportunities to shorten expiration times, the final step involves selecting those showings with input conflicts 710, and sorting these showings as in the first conflict resolution phase 711. The viewer is then presented with the option to cancel each previously scheduled recording in favor of the desired program 712, 713. Of course, the viewer may ultimately decide that nothing new will be recorded 714.

In a preferred embodiment of the invention, all conflicts are resolved as early as possible, giving the viewer more control over what is recorded. When the viewer makes an explicit selection of a program to record, the algorithm described in FIG. 7 is used to immediately schedule the recording and manage any conflicts that arise.

Once an explicit selection has been made, and the viewer informed that the recording will be done, it will not be canceled without explicit approval of the viewer.

Fuzzy recordings are periodically scheduled by a background task on the client device. Given the prioritized list of preferred programs as described earlier, the background scheduler attempts to schedule each preferred program in turn until the list is exhausted or no further opportunity to record is available. A preferred program is scheduled if and only if there are no conflicts with other scheduled programs. A preferred program which has been scheduled may be deleted under two conditions: first, if it conflicts with an explicit selection, and second, if a change in viewer preferences identifies a higher priority program that could be recorded at that time.

A further complication arises when handling aggregate viewing objects for which recording is requested. If conflict resolution was handled according to the method above for such objects, a potentially large number of conflicts might be generated, leading to a confusing and frustrating experience for the viewer in resolving the conflicts. Thus, when aggregate objects are chosen for recording, conflicts are automatically resolved in favor of the existing schedule.

In a preferred embodiment of the invention, conflicts resulting from the recording of aggregate objects will be resolved using the preference weighting of the programs involved; if multiple conflicts are caused by a particular program in the aggregate object, it will only be recorded if its preference exceeds that of all conflicting programs.

Methods Applied to Software Objects

The client system requires a complex software environment for proper operation. An operating system manages the interaction between hardware devices in the client and software applications which manipulate those devices. The television viewing object database is managed by a distinct software application. The time-warping software application is yet another application.

It is desirable to add new features or correct defects in these and other software subsystems which run on the client hardware device. Using the methods described herein, it is possible to replicate viewing objects containing updated software modules into the client system database. Once present in the client system database, the following unique and novel methods are used to install the updated software and cause the client system to begin executing the new software.

The software environment of the device is instantiated as a sequence of steps that occur when power is first applied to the device, each step building up state information which supports proper application of the following step. The last step launches the applications which manage the device and interact with the viewer. These steps are:

-   1. A read-only or electrically programmable memory in the device     holds an initial bootstrap sequence of instructions. These     instructions initialize low-level parameters of the client device,     initialize the disk storage system, and load a bootstrap loader from     the disk into memory, to which execution is then passed. This     initial bootstrap may be changed if it resides in an electrically     programmable memory. -   2. The second stage boot loader then locates the operating system on     the disk drive, loads the operating system into memory, and passes     execution to the operating system. This loader must exist at a     specific location on the disk so as to be easily located by the     initial loader.

The operating system performs necessary hardware and software initialization. It then loads the viewing object database software from the disk drive, and begins execution of the application. Other application software, such as the time-warping software and viewer interaction software, are also loaded and started. This software is usually located in a separate area on the disk from the object database or captured television programs.

Ideally, new software would be installed by simply copying it to the appropriate place on the disk drive and rebooting the device. This operation is fraught with danger, especially in a home environment. Power may fail while copying the software, resulting in an inconsistent software image and potential operating problems. The new software may have defects which prevent proper operation. A failure may occur on the disk drive, corrupting the software image.

Although the methods of this invention have referred to a disk drive, one skilled in the art will readily appreciate that the methods described here apply generally to any persistent storage system. A disk drive, and other persistent storage systems, are typically formatted into a sequence of fixed-size blocks, called sectors. “Partitions” are sequential, non-overlapping subsets of this sequence which break up the storage into logically independent areas.

With respect to FIG. 8, the invention maintains a sector of information at a fixed location on the disk drive 803 called the “boot sector” 804. The boot sector 804 contains sufficient information for the initial bootstrap 801 to understand the partitioning of the drive 803, and to locate the second stage boot loader 806.

The disk is partitioned into at least seven (7) partitions. There are two (2) small partitions dedicated to holding a copy of the second stage boot loader 806, two (2) partitions holding a copy of the operating system kernel 807, two (2) partitions containing a copy of the application software 808, and a partition to be used as scratch memory 809. For duplicated partitions, an indication is recorded in the boot sector 805 in which one of the partitions is marked “primary”, and the second is marked “backup”.

One skilled in the art will readily appreciate that, although two partitions are described herein for redundancy, triple, quadruple or greater degrees of redundancy can be achieved by creating more duplicated partitions.

With respect to FIGS. 9 a and 9 b, on boot 901, the initial bootstrap code reads the boot sector 902, scans the partition table and locates the “primary” partition for the second stage boot loader. It then attempts to load this program into memory 903. If it fails 904, for instance, due to a failure of the disk drive, the boot loader attempts to load the program in the “backup” partition into memory 905. Whichever attempt succeeds, the boot loader then passes control to the newly loaded program, along with an indication of which partition the program was loaded from 906.

Similarly, the second stage boot loader reads the partition table and locates the “primary” operating system kernel 907. If the kernel can not be loaded 908, the “backup” kernel is loaded instead 909. In any case, control is passed to the operating system along with an indication of the source partition, along with the passed source partition from above 910.

Finally, the operating system locates the “primary” partition containing application software and attempts to load the initial application 911. If this fails 912, then the operating system locates the “backup” partition and loads the initial application from it 913. An indication of the source partition is passed to the initial application, along with the source partition information from the previous steps. At this point, application software takes over the client system and normal viewing management behavior begins 914.

This sequence of operations provides a reasonable level of protection from disk access errors. It also allows for a method which enables new software at any of these levels to be installed and reliably brought into operation.

An “installer” viewing object in the object database is used to record the status of software installation attempts. It records the state of the partitions for each of the three levels above, including an indication that an attempt to install new software is underway 915. This operation is reliable due to the transactional nature of the database.

Referring to FIG. 10, installing a new software image at any of the three levels is handled as follows: the new software image is first copied into the appropriate backup partition 1001, and an indication is made in the database that a software installation is underway 1002. The primary and backup partition indications in the partition table are then swapped 1003, and the system rebooted 1004. Eventually, control will be passed to the initial application.

Referring again to FIG. 9 b, the first task of this application is to update the installer object. For each level 921, 922, the application checks if an installation was in process 916, 917, and verifies that the level was loaded off of the primary partition 918. If so, the installation at that level was successful, and the installer object is updated to indicate success for that level 919. Otherwise, the application copies the backup partition for that level over the primary partition and indicates failure in the installer object for that level 920. Copying the partition insures that a backup copy of known good software for a level is kept available at all times.

In a preferred embodiment of the invention, finalization of the installation for the top application level of software may be delayed until all parts of the application environment have been successfully loaded and started. This provides an additional level of assurance that all parts of the application environment are working properly before permanently switching to the new software.

Methods Applied to Operations Status Objects

Operations status objects are a class of viewing object in which information about the usage, performance and behavior of the client system is recorded. These objects are collected by the central site whenever communication with the central site is established.

The following operations status indicators are recorded for later collection along with a time stamp:

-   1. Viewer actions, primarily pressing buttons on a remote control     device, are recorded. Each “button press” is recorded along with the     current time, and any other contextual information, such as the     current viewer context. Post-processing of this object at the     central site results in a complete trace of viewer actions,     including the context in which each action is taken. -   2. Automatic actions, such as beginning or ending the recording of a     program, or choosing a program to record based on viewer     preferences, are recorded. In addition, deletion of captured     programs is recorded. Post-processing of this object at the central     site results in a complete trace of program capture actions taken by     the client system, including the programs residing in the persistent     store at any point in time. -   3. Software installation actions, including reception, installation,     and post-reboot results are recorded. -   4. Hardware exceptions of various kinds, including but not limited     to: power fail/restart, internal temperature profile of the device,     persistent storage access errors, memory parity errors and primary     partition failures.

Since all actions are recorded along with a time stamp, it is possible to reconstruct the behavior of the client system using a linear time-based ordering. This allows manual or automatic methods to operate on the ordered list of events to correlate actions and behaviors. For instance, if an expected automatic action does not occur soon after rebooting with new software, it may be inferred that the new software was defective.

Processing of Television Viewing Objects by Central Site Systems

Sources of Television Viewing Objects

A client system has a single source of television viewing objects: the central site. The central site object database has many sources of television viewing objects:

-   1. Program guide information obtained from outside sources is     processed to produce a consistent set of program guide objects,     indicating “programs”, “showings”, “channels”, “networks” and other     related objects. This set of objects will have dependencies     (“channels” depend on “networks”, “showings” depend on “programs”)     and other interrelationships. When a complete, consistent set of     objects is ready, it is added to the database as an atomic     operation. -   2. New software, including new applications or revisions of existing     software, are first packaged into “software” viewing objects. As     above, the software may have interdependencies, such as an     application depending on a dynamically loaded library, which must be     reflected in the interrelationships of the software objects     involved. In another example, there may be two types of client     systems in use, each of which requires different software objects;     these software objects must have attributes present indicating the     type of system they are targeted at. Once a consistent set of     objects is available, it is added to the database as an atomic     operation. -   3. Each client system has a unique, secret key embedded within it.     The public key matching this secret key is loaded into a “client”     management object, along with other interesting information about     the client, such as client type, amount of storage in the system,     etc. These objects are used to generate authentication objects as     necessary. -   4. Aggregation program guide objects are added in a similar fashion.     In this case, however, the aggregation object must refer to     primitive program guide objects already present in the database.     Also attached to the aggregation object are other objects, such as a     textual description, a screen-based icon, and other informational     attributes. Once a consistent set of ancillary objects to the     aggregation is available, it is added to the database as an atomic     operation. -   5. Data collected from client systems.

It should be clear that there may be any number of sources of viewing objects, and this enumeration simply shows the most basic possible sources.

Operations on Television Viewing Objects

There are a large number of possible operations on the central television viewing object database. The following examples are meant to show the type of processing that may be performed, however the potential operations are not limited to these examples:

-   1. Using various viewing objects, a number of interesting     statistical analysis tasks may be performed:     -   1.1. By examining large numbers of uploaded operations status         objects, it is possible to perform extensive analysis of         hardware reliability trends and failure modes. For instance, it         is possible to correlate internal temperature with expected MTBF         (Mean Time Between Failures) of client devices.     -   1.2. By examining large numbers of uploaded viewing information,         it is possible to derive demographic or psychographic         information about various populations of client devices. For         example, it is possible to correlate TV programs most watched         within specific zip codes in which the client devices reside.     -   1.3. Similarly, by examining large numbers of viewing         information objects, it is possible to generate “rating” and         “share” values for particular programs with fully automated         methods, unlike existing program rating methods.     -   1.4. There are many other examples of statistical analysis tasks         that might be performed on the viewing object database; these         examples are not meant to limit the applicability of the         invention, but to illustrate by example the spectrum of         operations that might be performed. -   2. Specialty aggregation objects may be automatically generated     based on one or more attributes of all available viewing objects.     -   Such generation is typically performed by first extracting         information of interest from each viewing object, such as         program description, actor, director, etc., and constructing a         simple table of programs and attributes. An aggregate viewing         object is then generated by choosing one or more attributes, and         adding to the aggregate those programs for which the chosen         attributes match in some way.     -   These objects are then included in the slices generated for         transmission, possibly based on geographic or other information.         Some example aggregates that might be created are:     -   2.1. Aggregates based on events, such as a major league football         game in a large city. In this case, all programs viewable by         client devices in or around that city are collected, and the         program description searched for the names of the teams playing,         coaches names, major player's names, the name of the ballpark,         etc. Matching program objects are added to the aggregate, which         is then sliced for transmission only to client devices in         regions in and around the city.     -   2.2. Aggregates based on persons of common interest to a large         number of viewers. For instance, an aggregate might be         constructed of all “John Wayne” movies to be broadcast in the         next week.     -   2.3. Aggregates based on viewing behavior can be produced. In         this case, uploaded viewing objects are scanned for elements of         common interest, such as types of programs viewed, actual         programs viewed, etc. For example, a “top ten list” aggregate of         programs viewed on all client devices in the last week might be         generated containing the following week's showing of those         programs.     -   2.4. Aggregates based on explicit selections by viewers. During         viewing of a program, the viewer might be presented with an         opportunity to “vote” on the current program, perhaps on the         basis of four perceived attributes (storyline, acting,         directing, cinematography), which generates viewing objects that         are uploaded later. These votes are then scanned to determine an         overall rating of the program, which is transmitted to those who         voted for their perusal.     -   2.5. There are many other examples of how the basic facilities         of this invention allow the service operator to provide         pre-sorted and pre30 selected groups of related programs to the         user of the client device for perusal and selection. These         examples are not meant to limit the applicability of the         invention, but to illustrate by example the spectrum of         operations that might be performed. -   3. Manual methods may also be used to generate aggregate objects, a     process sometimes called “authoring”. In this case, the person     creating the aggregate chooses programs for explicit addition to the     aggregate. It is then transmitted in the same manner as above.

Clearly, aggregation program objects may also permit the expression of preferences or recording of other information. These results may be uploaded to the central site to form a basis for the next round of aggregate generation or statistical analysis, and so on.

This feedback loop closes the circuit between service provider and the universe of viewers using the client device. This unique and novel approach provides a new form of television viewing by providing unique and compelling ways for the service provider to present and promote the viewing of television programs of interest to individuals while maintaining reliable and consistent operation of the service.

Methods for Secure Object Creation, Transmission. and Verification Using a Cryptographically Signed Filesystem

The invention provides a client system that performs the functions of a Digital Video Recorder (DVR) such as scheduling, recording, and playing back (using VCR-like actions) audio/video content (television broadcasts, satellite-TV broadcasts, cable-TV broadcasts, and direct downloads of program materials via the Internet).

The invention also provides a cryptographically signed file system that allows a secure central site to digitally sign software and data objects. These digitally signed software and data objects are verified by the client system to prevent any attempts to bypass the client system's normal operation using unauthorized software and/or data.

The invention's cryptographically signed filesystem uses a number of well-studied cryptographic algorithms and techniques. Hashing algorithms such as MD5 and SHA-1 have been extensively used, studied, and documented. SHA-1, the algorithm used in a prefened embodiment of the invention, is defined in Federal Information Processing Standard FIP180-1. Its predecessor algorithm SHA (nearly identical) is discussed in detail in Bruce Schneier's Applied Cryptography 2^(nd) edition (ISBN 0-471-11709-9, pages 442-445). SHA-1 is unpatented.

Digital signature algorithms such as RSA and EIGamal have also been widely used, studied, and documented. The EIGamal algorithm used in a preferred embodiment of the invention is discussed in Applied Cryptography (pages 476-479). This algorithm itself was never specifically patented, but may have been covered under the Diffie-Helman patent #4,200,770 (issued Apr. 29, 1980, expired Apr. 29, 1997).

The technique of combining hashing algorithms (MD4, MD5, SHA-1) with digital signature algorithms (RSA, EIGamal, etc.) has a long history of use, due to the fact that digital signature algorithms are comparatively slow and cannot handle large amounts of data efficiently. It is thus much more efficient to sign a hash, than to sign a message or piece of data directly. Discussions can be found in Applied Cryptography, and practical examples can be found in the Pretty Good Privacy (PGP) software and in most other “secure” electronic mail systems.

The idea of using a two level hash set (many hashes of individual pieces of data, combined into a collection which is itself hashed) with a single digital signature has been used within the 3DO videogame system. In this implementation, individual CD-ROM data sectors were hashed, the hashes were collected into a single large file, the result was hashed, and the final hash was signed using the RSA algorithm.

The invention incorporates some of the following features:

-   -   1. In a DVR product as described above and in FIG. 8, the DVR         software is structured and executed in a somewhat hierarchical         fashion. A DVR has a small amount of software, used for system         startup and self-test, located in a read-only memory chip         (“ROM”) 801 on its printed circuit board, and a much larger         amount of software located in a specially-designated portion 804         of the system's disk drive 803. Typically, the ROM software 801         performs some initial configuration of the system's hardware,         locates the disk-based software 804, loads a portion of the         disk-based software into memory 802, and executes it. The         disk-based software 804 performs additional initialization,         loads and executes other software from disk 803, may download         and execute software from an Internet-based server or from a         satellite system, and implements the actual DVR features. This         process, is usually referred to as “bootstrapping” or “booting”.     -   2. It is possible for any piece of computer software (in a DVR,         personal computer, network server, or elsewhere) to be         represented as a sequence of “bytes” or “characters” of numeric         data.     -   3. There exist algorithms which can compute a “hash” or         “fingerprint” of any sequence of numeric bytes. The “hash” which         results from an algorithm of this sort is a relatively short         sequence of bytes (typically 16 to 20 bytes) which is         characteristic of the specific sequence of bytes fed into the         algorithm. In a good algorithm of this sort, making any change         at all in the input sequence tends to result in a         chaotic-looking change in the output hash value. Typically,         changing even a single bit in the input causes roughly half of         the bits in the output hash to change. As the result of this         deliberately-chaotic behavior, it is extremely difficult to find         any way to alter an input sequence which will not cause the         corresponding hash value to change—making any such change         successfully would require many years of trial-and-error effort         by the fastest computers available today. A number of algorithms         of this sort are in wide use today and have been extensively         studied by the cryptographic community—the MD5 and SHA-1         algorithms are in this class. These algorithms are fast and         efficient—it is possible to compute the 20-byte SHA-1 hash value         of a megabyte of data in less than a second, on even a         relatively slow processor.     -   4. There exist algorithms which can “sign” a piece of numeric         data, and allow the signature to be verified with high         reliability by another party.

The “signature” is the output of a mathematical function, which takes as its input the original numerical data and a “private key” (a number known only to the party computing the signature). The signature can be verified using another mathematical function, which takes as its input the original numerical data, the signature, and a “public key”. The public key is a number which is mathematically related to the private key, but which cannot be used in any reasonable amount to compute the private key, and cannot itself be used to “sign” data. Well-studied algorithms of this sort include RSA and EIGamal. Signing and verification is a compute-intensive task—computing or verifying a signature may require seconds, or even tens of seconds on a slow processor.

Method

The invention combines “hashing” and “signature” algorithms within the framework of a phased, hierarchical “bootstrap” operation, in order to perform a complete validity check of all software on a DVR while requiring only a single (computationally-expensive) signature verification.

In this method, the software executed by the DVR is grouped into four categories as previously described above and in FIG. 8:

-   -   1. Startup software in the DVR's ROM chip 801. In a preferred         embodiment of the invention this consists of initial hardware         configuration code, system self-test code, and code to implement         the SHA-1 algorithm. This code is programmed into the ROM when         the system is manufactured in the factory, and is never intended         to be modified. If the system's ROM is actually implemented         using a “flash ROM” chip (which can be erased and rewritten), it         is necessary for this startup code to be located entirely within         a portion of the flash chip which can be “locked” (physically         and permanently secured against erasure or modification).     -   2. Second-level “monitor” code in the DVR's ROM chip or disk         drive 806. In a preferred embodiment of the invention, this         consists of further initialization code, code to support system         debugging, code capable of accessing the DVR's disk drive and         “bootstrapping” the software in the next category, the         mathematical code needed to verify a digital signature, and a         set of one or more “public” keys associated with “trusted”         entities. This software is typically programmed into the ROM or         disk drive when the DVR is manufactured, and is not intended to         be changed thereafter. If the system's ROM is implemented using         a “flash ROM” chip, the second-level monitor code does not need         to be located in a permanently-locked portion of the flash chip.     -   3. Software and data “bootstrapped” into memory from the disk         drive 803 by the ROM. In a preferred embodiment of the         invention, this consists of the “kernel” of the Linux operating         system 807, and a copy of a small “ramdisk” filesystem which         holds one executable program and one or more files of data. This         software is pre-loaded onto the disk drive 803 when the system         is manufactured. It can be updated, upgraded, or replaced when         the DVR system is in use in a customer's home, via a software         download from an Internet server or satellite system, as         described above.     -   4. Software and data loaded into memory from the disk drive 803         by the operating system. In a preferred embodiment of the         invention, this consists of all of the software which implements         the DVR's basic and enhanced functions, and consists of several         hundred individual files in the “root” filesystem 808. As with         the kernel, this software is normally installed at the factory,         and can be upgraded via a software download.

A fundamental aspect of this method can be summarized as follows: “The software in each category, is responsible for verifying the integrity of all of the software in the next category before that software is allowed to be trusted or executed.” This is described above and in FIG. 9.

Referring to FIG. 11, in order to implement this verification process, the software in each category is “fingerprinted” using a high-quality hashing algorithm by a secure release server. In a preferred embodiment of the invention, the SHA-1 algorithm is used. Three different hashes (or sets of hashes) are used to create the software and data used by the client system DVR:

-   -   1. The second-level “monitor” code in the ROM is hashed 1101, at         the time that the ROM software is manufactured. The resulting         hash value is stored as part of the first-level “startup”         software in the ROM, in the permanently-locked portion of the         ROM.     -   2. The code and data being “bootstrapped” from disk is hashed         1104, and a digital signature is computed using a secure private         key. In the preferred implementation, the hashing is performed         on the Linux kernel and the ramdisk filesystem, and the EIGamal         digital signature algorithm is used to compute the signature.         The hash value, and the digital signature are stored as part of         the bootstrapped information on disk.     -   3. Each file in the root filesystem is hashed individually 1102.         In a preferred embodiment of the invention, a “hash table” file         is constructed 1103—each line in this file gives the name of a         file in the root filesystem, and the hash value of the file's         contents. This hash table file is combined with an executable         program, and the two files are used to construct the “ramdisk”         filesystem image which is then paired with the Linux operating         system kernel.

Once all of these hashes, and the digital signature are created, the software can be installed on a DVR.

With respect to FIGS. 12 a and 12 b, when the DVR is powered on, the software goes through the following series of steps:

-   -   1. The system's central processor automatically starts execution         at a specific place in the ROM code 1201. The initial ROM         execution is located within a portion of the chip's memory which         cannot be modified (e.g., within the “locked” region of a flash         memory chip).     -   2. The ROM code performs some low-level initialization of the         system's hardware 1201.     -   3. The ROM code executes its built-in SHA-1 hashing function,         using as input the contents of the second-level “monitor”         portion of the PROM. It compares the result of this hashing         calculation with the “official” SHA-1 result calculated by the         DVR's manufacturer 1202. If the results do not match 1203, the         ROM “knows” that the second-level “monitor” code has been         tampered with, and the ROM disables the system without executing         the code that has been altered 1204. This hash verification         takes a small fraction of a second.     -   4. If the second-level monitor code has not been altered, the         ROM code executes the second-level monitor 1205.     -   5. The second-level monitor code accesses the system's hard         drive, and loads the “bootstrapped” code and data into memory         1206. In a preferred embodiment of the invention, this consists         of the Linux kernel, the image of the “ramdisk” filesystem, and         a small block of data which contains the SHA-1 hash and the         EIGamal digital signature of the hash.     -   6. The second-level monitor code uses the built-in SHA-1 hashing         function to compute the hash value of the bootstrapped code just         loaded 1206. It then compares the result of this hashing with         the SHA-1 hash value attached to the bootstrapped code. If the         two hash values do not match 1207, the monitor “knows” that the         bootstrapped code has been tampered with, accidentally         corrupted, or altered in some other way, and disables the system         without executing the bootstrapped code 1208. This hash         verification takes approximately one second.     -   7. The second-level monitor code extracts the digital signature         from the bootstrapped data. It examines the signature and         matches the “key name” in the signature against the names in the         public keys it is allowed to trust 1206—if the signature was         generated using a key it doesn't recognize 1207, it disables the         system without executing the bootstrapped code 1208. It executes         the code necessary to verify that the signature was actually         calculated by someone in possession of the correct private         key—if the signature cannot be verified, it disables the system         without executing the bootstrapped code. The signature         verification requires roughly 20 seconds in the preferred         implementation.     -   8. Once the signature has been verified, the second-level         monitor code copies the bootstrapped data to a reserved area in         memory, and then executes the bootstrapped code 1209. In the         preferred implementation, the code being executed is the Linux         kernel.     -   9. The bootstrapped code locates the bootstrapped data in         memory, and accesses its contents. In a preferred embodiment of         the invention, the data contents consists of the “ramdisk”         filesystem, and contains one executable program, the “valid hash         table” file, and (optionally) one or more small files. The Linux         kernel executes the program (filesystem scanning software)         contained within this filesystem 1210.     -   10.The executable program accesses the “root” filesystem on the         hard drive. It scans every file in the root filesystem,         calculates the hash value for the file, and confirms that the         file name and hash value match an entry in the “valid hash         table” in the bootstrapped data 1210, 1211. If any mismatches         are found—if any file isn't listed in the hashtable, or if it's         listed but with a different hash value—the scanning program         takes corrective action 1212. In a preferred embodiment of the         invention, the typical corrective action is to erase the         mismatching file from the root filesystem, or to replace the         mismatching file by a valid “backup copy” contained within the         “ramdisk” filesystem. In a preferred embodiment of the         invention, scanning and verification of a typical “root”         filesystem requires approximately 30 seconds.     -   11. Once all files in the root filesystem have been hashed and         verified, the scanning program exits, and the system begins         executing programs from the root filesystem 1213. At this point,         all files in the root filesystem can be assumed to be valid—any         files which have been added or altered, will have been detected         with very high reliability and will have been removed from the         system or replaced by backup copies from the ramdisk filesystem.         Updating the DVR Software

Steps 1102, 1103, and 1104 as described above, are executed when software or data object updates are created by the release server. Referring again to FIG. 1, the file objects 106 are distributed to the central site 100. The objects 106 are sliced 102 and sent to distribution servers 110. The transmission of the sliced objects are performed as described above.

The initial-startup software in the system ROM is not intended to be modified after the DVR has been manufactured. In a preferred embodiment of the invention, it is stored in a portion of the system's memory chip which is truly “read only” and is physically incapable of being modified. To replace this portion of the DVR software, it is necessary to remove the ROM chip and replace it—a somewhat tricky and demanding task, which can be made more difficult through physical means (e.g., gluing the ROM chip to the DVR's printed circuit board).

The second-level “monitor” code in the system's ROM is not intended to be modifiable. Although it may be physically located in a region of a flash-memory chip or disk drive which can be physically altered, any attempt to do so would alter the SHA-1 hash value of this region of the chip. The initial-startup code would detect the change to the SHA-1 hash, and disable the system.

The bootstrapped code and data on disk, and the “root” filesystem, are intended to be modifiable, but only in a secure fashion. In order to modify any of this disk-based information, and have the DVR remain usable, the person performing the modification must possess a “private” key which corresponds to one of the “public” keys accepted by the second-level “monitor” code. A complete and consistent set of hash values must be computed, a correct digital signature calculated, and all of the files and hashes and signatures updated at the same time. Any attempt to alter the bootstrapped code or data, or the root filesystem, without going through the proper hashing-and-signing procedure will result in an inconsistent set of hash and signature values, and the DVR will either not boot at all, or will delete any files in the root filesystem which don't match its hash table.

Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the claims included below. 

1. A process for a cryptographically signed filesystem that ensures that authorized software and data are used by a digital video recorder (DVR), comprising the steps of: confirming a hash value of a bootstrap code on a persistent storage device on said DVR; disabling said DVR when said hash value is not confirmed; extracting and verifying a digital signature of said bootstrap code if said hash value is confirmed; transferring control to said bootstrap code when said digital signature is verified; disabling said DVR when said digital signature is not verified; scanning each file in a root filesystem on said persistent storage device to validate that each file name and hash value of each file matches an entry in a hash table file; and automatically replacing a file by said scanning step that said scanning step has found does not correctly match an entry in said hash table file with a valid backup file from said persistent storage device.
 2. The process of claim 1, further comprising the steps of: executing, upon bootup of said DVR, a startup software resident in read only memory on said DVR; wherein said startup software verifies a hash value of a second stage loader software; transferring control to said second stage loader when said second stage loader's hash value is valid; disabling said DVR when said second stage loader's hash value is not valid; and wherein said second stage loader confirms the hash value of said bootstrap code.
 3. The process of claim 2, further comprising the steps of: calculating a hash value for said second stage loader; and storing said calculated hash value as part of said startup software in said read only memory on said DVR.
 4. The process of claim 1, wherein said hash table file is stored on said persistent storage device and contains valid hash values for each file in said root filesystem.
 5. The process of claim 4, wherein if any files in said root filesystem are found to be invalid, then the invalid files are deleted.
 6. The process of claim 4, wherein if any files in said root filesystem are found to be invalid, then the invalid files are replaced with valid copies.
 7. The process of claim 1, further comprising the steps of: calculating a hash value for said bootstrap code on a server; creating a digital signature for said bootstrap code on said server; and storing said bootstrap hash value and said bootstrap digital signature as part of said bootstrap code.
 8. The process of claim 7, further comprising the step of: updating said DVR with said bootstrap code from said server.
 9. The process of claim 1, further comprising the steps of: creating a hash table file on a server; calculating a hash value on said server for each file in said root filesystem; entering each file's file name and hash value in said root filesystem into said hash table file; and incorporating said hash table file into said bootstrap code.
 10. The process of claim 9, further comprising the step of: updating said DVR with said bootstrap code and said root filesystem from said server.
 11. An apparatus for a cryptographically signed filesystem that ensures that authorized software and data are used by a digital video recorder (DVR), comprising: a persistent storage device on said DVR; a bootstrap code on said persistent storage device; a module for confirming a hash value of said bootstrap code; a disabling module that disables said DVR when said hash value is not confirmed by said confirming module; a module for extracting and verifying a digital signature of said bootstrap code if said hash value is confirmed; wherein said extracting and verifying module transfers control to said bootstrap code when said digital signature is verified by said extracting and verifying module; a disabling module that disables said DVR when said digital signature is not verified by said extracting and verifying module; a module for scanning each file in a root filesystem on said persistent storage device to validate that each file name and hash value of each file matches an entry in a hash table file; and wherein said scanning module automatically replaces a file that said scanning module does not correctly match an entry in said hash table file with a valid backup file from said persistent storage device.
 12. The apparatus of claim 11, further comprising: a startup module resident in read only memory on said DVR; wherein said startup module is executed upon bootup of said DVR; a second stage loader module; wherein said startup module verifies a hash value of said second stage loader; wherein said startup module transfers control to said second stage loader when said second stage loader's hash value is valid; and a disabling module that disables said DVR when said second stage loader's hash value is not valid.
 13. The apparatus of claim 12, further comprising: a module for calculating a hash value for said second stage loader; and a module for storing said calculated hash value as part of said startup software in said read only memory on said DVR.
 14. The apparatus of claim 11, wherein said hash table file is stored on said persistent storage device and contains valid hash values for each file in said root filesystem.
 15. The apparatus of claim 14, wherein if any files in said root filesystem are found to be invalid, then said performing a scan module deletes the invalid files.
 16. The apparatus of claim 14, wherein if any files in said root filesystem are found to be invalid, then said performing a scan module replaces invalid files with valid copies.
 17. The apparatus of claim 11, further comprising: a module for calculating a hash value for said bootstrap code on a server; a module for creating a digital signature for said bootstrap code on said server; and a module for storing said bootstrap hash value and said bootstrap digital signature as part of said bootstrap code.
 18. The apparatus of claim 17, further comprising: a module for updating said DVR with said bootstrap code from said server.
 19. The apparatus of claim 14, further comprising: a module for creating a hash table file on a server; a module for calculating a hash value for each file in said root filesystem on said server; a module for entering each file's file name and hash value in said root filesystem into said hash table file; and a module for incorporating said hash table file into said bootstrap code.
 20. The apparatus of claim 19, further comprising: a module for updating said DVR with said bootstrap code and said root filesystem from said server. 